The present invention relates to a receiver and a receiving method and, in particular, but not exclusively to a spread spectrum receiver and method of receiving spread spectrum signals.
In wireless cellular telecommunication networks, the network is divided into a plurality of cells each of which is served by a base station. Each base station is arranged to communicate with mobile terminals in the cell associated with the base station. Spread spectrum techniques have been proposed for use with cellular networks. For example, the IS-95 standard in the USA uses a direct sequence spread spectrum technique which is often referred to as a code division multiple access (CDMA) technique. In these systems, a plurality of users in the different cells use the same frequency at he same time. The users are generally distinguished by different spreading codes. The capacity and signal quality of such a system is strongly influenced by the amount of interference and how effectively interference can be removed from received signals. If the signal processing can be improved to further remove interference, a greater degree of interference can in practice be tolerated which will in turn lead to a potential increase in capacity and/or signal quality.
One type of interference which arises is xe2x80x9cnarrow band interferencexe2x80x9d. In other words, the bandwidth of the interference is much less than the frequency band over which the signal is spread. The inherent processing gain of spread spectrum systems may provide some protection against narrowband interference. However, in some situations, the power level of that narrow band interference is sufficiently high that the system is not able to provide protection against that interference.
In an article entitled xe2x80x9cA Novel Technique For Narrowband/Broadband Interference Excision in DS-SS Communications (Proceedings of Milcom 1994 Conference pages 628-632), E Kanterakis discusses a technique for reducing the narrowband interference. The technique described in this document relies on setting the magnitude response of the received sigrnal Fourier transform to a predetermined function while leaving the phase response unchanged. The Fourier transform magnitude response is made as constant as possible over the entire signal spectrum to thereby provide a whitening filter. In this document, amplitude response is set to a magnitude response which is representative of a single bit of the received signal with interference.
It is therefore an aim of embodiments of the present invention to provide a receiver which is capable of providing an improved narrowband interference suppression.
According to one aspect of the present invention there is provided a receiver comprising input means for receiving a signal of a first domain; processor means for applying a transform function to the received signal to transform said signal to a second domain; a first extractor arrangement for extracting a firs: type of information from the signal in the second domain; a plurality of processor means, each of which is arranged to receive a plurality of information portions, apply said transform function thereto and provide a transformed output at each of said plurality of processor means; a second extractor arrangement for extracting a second, different type of information from the output of each of said plurality of processor means; and compensation circuitry for compensating the received signal using one of the second type of information extracted from one of the outputs of the to plurality of processor means.
In practice, the arrangement defined hereinbefore which uses a plurality of information portions provides significantly better results than the arrangement of the document mentioned previously.
The input means may receive a spread spectrum signal. The spread spectrum signal may be a direct sequence spread spectrum signal. The spread spectrum signal may be a hybrid signal which Includes a direct sequence signal combined, for example, with a frequency hopped technique.
The first domain may be time domain. The second domain may be the frequency domain.
The processor means may use a Fourier transform function to convert the signal of the first domain to a signal in a second domain. Alternatively, any other suitable transform function may be used.
The first type of information may be phase information whilst the second type of information may be amplitude information.
Preferably, the information portions comprise bits. However, this will depend on the format of the transmitted signal. Preferably, each of the plurality of processor means is arranged to receive two bits. However, the plurality of processor means may be arranged each to receive more than two bits.
The number of processor means may be equal to the maximum number of different outputs which the second extractor arrangement is able to provide. For example, in one embodiment of the present invention, two bits may be used. Four possible is combinations are available. However, in practice, only two combinations may need to be used as the outputs of two of the combinations may be the same and the output of the other of the two combinations may be the same. In a preferred embodiment of the present invention, the number of processor means is equal to two.
The compensation circuitry may comprise means for combining the output of the first extractor arrangement with each of the outputs of the second extractor arrangement to provide a plurality of signals. The compensation circuitry may comprise decision circuitry for deciding based on the plurality of signals provided by the combining means the likely information contained in the received signal. The decision circuitry may comprise means for applying an inverse of the transfer function to the plurality of signals output by said combining means.
The decision circuitry may comprise matched filters for matched filtering the output of the combining means or equivalent circuitry such as correlation circuitry for correlating the output of the combining means. The matched filters may be connected to the outputs of the means for applying an inverse of the transform.
The receiver described hereinbefore may be incorporated in a mobile station or a base station.
According to a second aspect of the present invention, there is provided a receiving method comprising the steps of receiving a signal in a first domain; applying a transform function to the received signal to transform said signal to a second domain; extracting a first type of information from the signal in the second domain; applying said transform function to a plurality of different information portions; extracting a second, different type of information from each of the plurality of different information portions to which the transform function has been applied; and compensating the received signal using said second type of information.